Present combustors used in gas turbine engines for powering aircraft in flight include radially outer and inner combustion liners and a single annular dome joining upstream ends thereof. The single dome includes a plurality of circumferentially spaced carburetors each including a fuel injector nozzle and a conventional air swirler for providing a fuel/air mixture into the combustor. The combustor has a burning length defined between the dome at the fuel injector nozzle to the leading edge of a conventional turbine nozzle disposed at the outlet of the combustor. The combustor also has a dome annulus height measured between the outer and inner liners at the dome end of the combustor.
Since the combustor is used in powering an aircraft in flight, it must operate over a wide range of power conditions from low power at ground idle to high power at takeoff, for example. Performance of the combustor is evaluated by several conventional parameters including the degree of uniformity of the combustion gas exit temperature, as represented by the conventionally known profile and peak pattern factors, efficiency of combustion, and the amount of exhaust emissions from low to high power operation. A relatively large length-to-height ratio is generally desirable for obtaining acceptable combustion gas exit temperature uniformity and relatively low unburned hydrocarbon and CO emissions. However, a relatively large length-to-height ratio results in a relatively long combustor which is generally undesirable for its relative increase in weight and surface area which requires cooling, and for the increased production of NO.sub.x emissions. Combustion gas residence time is the amount of time combustion occurs in the combustor and relatively long residence times reduce unburned hydrocarbons and CO but increase NO.sub.x production when at high temperature.
Accordingly, it is a primary objective in gas turbine engine combustor design to have relatively compact and short combustors which provide a good balance between competing objectives including reduced exhaust emissions, reduced weight, and acceptable exit temperature uniformity. Combustors which are too short result in undesirable and excessive gas temperatures for a given annulus height, for example, or flame instability, or both, where dome height and burning length are reduced excessively.
Improved gas turbine engine combustor concepts have been studied for improving efficiency thereof while obtaining reduced exhaust emissions among other objectives. One such study includes the National Aeronautics and Space Administration (NASA) Energy Efficient Engine (E.sup.3) program in which advanced, short length, double annular or double dome combustors were designed and evaluated. A double dome combustor, such as for example, the E.sup.3 combustor, includes two parallel radially outer and inner combustion zones each having a burning length-to-dome height ratio. The double dome combustor includes an outer dome having a plurality of circumferentially spaced outer carburetors therein, and an inner dome having a plurality of circumferentially spaced inner carburetors therein. Each of the length-to-height ratios is generally equal to conventional single dome length-to-height ratios for obtaining acceptable performance, while obtaining a relatively short combustor. For example, a double dome combustor can be sized for replacing a comparable single dome combustor having equivalent dome airflow in about half its length since if both the length and dome heights are reduced in half, the same length-to-height ratio can be obtained in half the length. Since each of the length-to-height ratios of the two combustion zones in the double dome combustor is generally equal to the length-to-height ratio of the corresponding single dome combustor, the equivalent exit temperature pattern factor can be achieved with a 50% reduction in combustor length. The combustion zone residence time is also reduced by about 50%.
Accordingly, conventional double dome combustors as studied in the literature can be effective for reducing overall combustor size while obtaining comparable or improved performance over the wide power range required during operation of an aircraft gas turbine engine.
However, various double dome combustor concepts are known in the literature which operate at varying degrees of efficiency and performance, and have different sizes. It is generally desirable to obtain yet further decreases in combustor length for further reducing weight and surface area, and therefore reducing cooling air requirements thereof, while still obtaining acceptable low to high power operation including reduced exhaust emissions and acceptable mixing of the combustion gases and dilution air for obtaining acceptably uniform combustion gas exit temperatures.